The influence of intermediate pressure on air-source scroll heat pump systems with gas-injected enthalpy enhancement is studied through experiments. The results show that the heating capacity and COP of the system first increase and then decrease with the increase of gas-injected pressure, indicating the existence of an optimal gas-injected pressure. Under different operating conditions, the optimal gas-injected pressure varies, but the corresponding refrigerant state in the gas-injected branch remains saturated. When the gas-injected pressure increases from the minimum value to the optimal gas-injected pressure, the heating capacity and COP improve by an average of 7.8% and 4.1%, respectively. Therefore, during the control of actual units operating in gas-injected mode, the superheat of the refrigerant in the gas-injected branch should be minimized to approach a saturated state to improve the efficiency of the units.